Commercially available display devices are classified into tube-type display devices, such as CRTs, and flat display devices, such as liquid crystal display devices, EL display devices, and plasma display devices. A flat display device is composed essentially of an aggregation of pixels, and display on it is controlled pixel by pixel so as to produce an overall image. In a flat display device, for example a liquid crystal display device, an array of thin-film transistors are formed on a glass substrate so that display pixels are driven individually by those transistors.
FIG. 11 shows the structure of a thin-film transistor (hereinafter referred to as a “TFT”) that is conventionally used in a liquid crystal display device. In the TFT shown in the figure, on top of a gate electrode G, a semiconductor layer SI of, for example, silicon is formed with an insulating film interposed in between, and then, further on top, a source electrode S and a drain electrode D, both rectangular in shape as seen in a plan view, are arranged side by side with a predetermined interval secured in between. Ideally, the source and drain electrodes S and D should be formed in the designed positions. Occasionally, however, they are formed in deviated positions as shown in FIGS. 12 and 13. If, as shown in FIG. 12, the source and drain electrodes S and D deviate in the vertical direction of the figure while maintaining equal overlaps with the gate electrode G, the areas (hatched in the figure) over which the source and drain electrodes S and D overlap the gate electrode G remain unchanged, and thus the parasitic capacitance of the TFT remains almost unchanged. However, if, as shown in FIG. 13, the deviation occurs in the horizontal direction of the figure, one of the source and drain electrodes S and D comes to overlap the gate electrode G over a larger area than the other. This results in a great variation in the parasitic capacitance.
Based on what has been described thus far, one can recognize the need to reduce variations in the parasitic capacitance of thin-film transistors, in particular those used to drive display pixels, and the need to achieve uniform image quality on a display device of which the display pixels are driven by transistors.